The present invention relates to a method for optimizing the operating lifetime of an energy storage system comprising a battery pack with a plurality of battery cells, said method comprising: measuring the temperature of the battery pack by means of a sensor unit and measuring at least one further parameter representing an indication of the operating lifetime of the battery pack.
The invention also relates to a system for optimizing the operating lifetime of an energy storage system comprising a battery pack with a plurality of battery cells, said system comprising a sensor unit for measuring the temperature of the battery pack and at least one further parameter representing an indication of the operating lifetime of the battery pack.
The invention can be applied in heavy-duty vehicles, such as trucks, buses and construction equipment. Although the invention will be described with respect to a bus, the invention is not restricted to this particular vehicle, but may also be used in other vehicles such as cars and working machines such as articulated haulers, excavators and backhoe loaders, and furthermore also boats and industrial applications in which batteries are used.
In the field of vehicles, there is a steady increase in research and development related to propulsion of vehicles with alternative power sources, i.e. power sources being used as alternatives to conventional internal combustion engines. In particular, electrically operated vehicles has emerged as a promising alternative.
According to today's technology, a vehicle can be operated by means of an electric machine solely or by means of an arrangement comprising both an electric machine and an internal combustion engine. The latter alternative is often referred to as a hybrid vehicle (HEV), and can for example be utilized in a manner in which an internal combustion engine is used for operating the vehicle while driving outside urban areas whereas the electric machine can be used in urban areas or in environments in which there is a need to limit the discharge of harmful pollutants such as for example carbon monoxide and oxides of nitrogen.
The technology involved in electrically operated vehicles is closely related to the development of electrical energy storage systems, such as battery-related technology for vehicles. Today's electrical energy storage systems for vehicles may comprise a battery pack with a plurality of rechargeable battery cells which, together with control circuits, form a system which is configured for providing electric power to an electric machine in a vehicle.
A vehicle which is operated by means of an internal combustion engine and an electric machine supplied with power from a rechargeable electrical energy storage system is sometimes referred to as a plug-in hybrid electric vehicle (PHEV). A plug-in hybrid electric vehicle normally uses an energy storage system with rechargeable battery cells which can be charged through a connection to an external electric power supply. During charging, a high amount of energy is fed into the energy storage system in a relatively short time in order to obtain fuel savings and to optimize the vehicle's range of driving.
For this reason, the actual charging of the energy storage system is suitably implemented through a process in which a control unit on the vehicle requests for a charging process to be carried out by means of an external electric power supply. This is carried out after the energy storage system and the external power supply have been electrically connected by means of suitable connector elements.
In the field of automotive technology, an energy storage system normally comprises a battery pack with a large number of battery cells. Using a plug-in hybrid vehicle as an example, a battery pack may for example be of the lithium-ion type. In the event that a 600 V lithium-ion battery pack is used, approximately 200 battery cells connected in series will then be needed to achieve a desired voltage in order to operate the vehicle. The available range for driving the vehicle depends on certain parameters such as the state of charge (SOC) of the battery pack.
Furthermore, it is known that batteries degrade over time and that the expected driving range and fuel savings of a vehicle cannot be upheld towards the end of the lifetime of a battery due to a decreasing performance of the battery. Also, the decreasing performance will affect the magnitude of the power which can be received and supplied by the battery.
Even though rechargeable batteries are capable of being repeatedly recharged and reused many times, it is still a fact that they have a limited lifetime. The point in time in which a battery approaches and finally reaches its “end of life” phase (EOL) can be said to correspond to a time when the performance requirements of the battery are no longer met. Consequently, there is a need for systems and methods for optimizing the lifetime of a rechargeable battery while considering the fact that it will eventually degrade and reach its end of life.
It is also known to use a battery management system in a vehicle in order to ensure safe operating conditions of the vehicle. In such a battery management system, there could be provided a sensor arrangement which is configured for measuring one or more parameters indicating the operation of the battery, for example in the form of a voltage sensor for each battery cell in order to measure the cell terminal voltage for each cell. Such a sensor arrangement could then be used for detecting various parameters related to the operation of the battery.
The patent document U.S. Pat. No. 8,773,066 teaches a system and method for extending the effective lifetime of a vehicle battery pack. The method comprises a step of operating the battery pack in a lifetime-extending storage mode during certain time periods, said storage mode being arranged for charging the battery pack to a lifetime-prolonging state of charge (SOC) which is less than the full state of charge in each charging session.
Furthermore, the patent document US 2014/079967 teaches a method and system arranged for the purpose of recovering a deteriorated output performance. Said method comprises a step of extending the lifetime of the battery pack by heating it during an over-discharge process.
Although there exist known systems and methods which are directed towards prolonging the lifetime of a vehicle battery pack, there is still a desire for more effective solutions within this field of technology.
Consequently, it is desirable to provide an improved method and system for optimizing the expected operating lifetime of an energy storage system, in particular for a battery pack for use within the vehicle industry, for solving the problems associated with prior solutions, in particular problems relating to a reduced power delivery capacity for the battery close to its end of life.
According to a first aspect of the invention, a method is provided for optimizing the operating lifetime of an energy storage system comprising a battery pack with a plurality of battery cells. The method comprises: measuring the temperature of the battery pack by means of a sensor unit; and measuring at least one further parameter representing an indication of the operating lifetime of the battery pack. Furthermore, the method comprises the steps of: detecting whether said parameter reaches a predetermined threshold value indicating that said battery pack is in a condition close to an end of its expected operating lifetime; and, in the event that said threshold value is reached, initiating heating of said battery pack, thereby increasing the temperature of said battery pack to a predetermined value so as to prolong said operating lifetime of the battery pack.
The invention provides certain advantages, in particular when the battery pack is about to reach its end of life, i.e. when the performance of the battery pack is decreasing. Due to the above-mentioned step of heating the battery pack, the internal resistance of the battery cells will be reduced, which leads to an increased power capacity of the battery pack. By decreasing the internal resistance at a suitable point in time when it has been detected that the power requirements are about to be insufficient, the operating lifetime can in fact be prolonged. In other words, the battery pack will then be able to meet the performance requirements for an additional period of time, as compared with a case in which no heating is initiated.
Generally, the invention is also based on the knowledge that the internal resistance of the battery cells increases as it ages. This also means that the power capacity of the battery pack decreases during said ageing process, i.e. the battery pack will have a reduced power capacity towards the end of its operative lifetime. However, by initiating heating of the battery pack at a stage when it is determined that the battery pack is in a condition close to an end of its expected operating lifetime, the internal resistance of the battery cells will be decreased. This means that the power capacity will increase for a further period of time. In this manner, the aim of the invention, i.e. to increase the operating lifetime of the battery pack, can be achieved.
It should be noted that the lifetime of the battery pack will in fact be shorter than a nominal lifetime if a heating process is initiated from start of its lifetime (due to the fact that heating increases the ageing processes of the battery cells), and for this reason it is important to initiate the increase in temperature as late in the life as possible and only to the extent to fulfill the power requirements of the battery pack.
According to a second aspect of the invention, a system is provided for optimizing the operating lifetime of an energy storage system comprising a battery pack with a plurality of battery cells. The system comprises a sensor unit for measuring the temperature of the battery pack and at least one further parameter representing an indication of the operating lifetime of the battery pack. Furthermore, the system comprises a control unit and a heating unit associated with said battery pack, wherein said control unit is configured for detecting whether said parameter reaches a predetermined threshold value indicating that said battery pack is in a condition close to an end of its expected operating lifetime; and also, in the event that said threshold value is reached, for initiating said heating unit for heating said battery pack, thereby increasing the temperature of said battery pack to a predetermined value so as to prolong said operating lifetime of the battery pack.
Further advantages and advantageous features of the invention are disclosed in the following description and in the dependent claims.